In the cellular communications market, the demand for high data rate communications continues to grow, and this growth is coupled with an exponential increase in wireless subscribers. These trends have led to the development and widespread use of spectrally efficient, complex modulation protocols for use with signals that have high peak to average power ratios (PAPR) and wide signal bandwidth. These complex modulation protocols, which are implemented in a transmitter lineup, pose significant challenges to conventional techniques for linearizing downstream radio frequency (RF) power amplifier operation.
Digital predistortion (DPD) techniques are commonly implemented to improve power amplifier linearization. More specifically, implementation of a DPD technique in a transmitter lineup may result in significant improvements to power amplifier efficiency by enabling the amplifier to be operated at a less significant backoff point from its saturation point, while meeting required linearity conditions (e.g., spectrum emission mask (SEM) and adjacent channel power (ACP) specifications).
However, the performance of conventional DPD techniques suffers significant degradation when used with complex modulation schemes that are characterized by high PAPR and wide signal bandwidth. The performance challenges increase when the RF power amplifier is being operated under dynamic conditions (e.g., when the amplifier is operating in an ETM2 (Enhanced Test Mode 2)) or 3G LTE-TDD (Long Term Evolution Time-Division Duplex) mode. Accordingly, developers continue to search for DPD apparatus and methods that result in improved power amplifier linearization even when used in conjunction with complex modulation schemes developed for signals with high PAPR and wide signal bandwidth.